The present invention relates to head positioning control systems in magnetic disc drives. More specifically, the present invention relates to a disc drive control system using a gray code and servo bursts provided in track sectors.
Computer systems, or the like, often times employ magnetic disc drives to store information such as computer programs or data. Magnetic disc drives typically include a transducing data head mounted on a slider which "flies" over the surface of a rotating rigid magnetic disc. The transducing data head is positioned over a selected portion of the disc by a drive controller operating an actuator. The data head is used to generate magnetic fields which are impressed onto the surface of the disc during writing of information, and to sense magnetic fields from the disc surface during readback of information.
Information is typically stored in concentric tracks on the surface of the magnetic disc by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the actuator so the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head.
Accurately positioning the data head over a track on the disc is of great importance in writing data to the disc and reading data from the disc. There are typically two steps in positioning the data head over the track. The first is referred to as coarse positioning, and the second is referred to as fine positioning. During coarse positioning, the servo system positions the head over a track on the disc based upon a track address position signal received from either a drive controller controlling the disc drive, or from a host system indicating data to be accessed. The data head is typically smaller than the distance between the tracks on the disc. Therefore, fine positioning is used to appropriately position the data head within the track over which it is positioned during coarse positioning.
When the data head is not properly located within the desired track, it is said to be "off track." The further off track the data head when either writing or reading, the larger the noise to signal ratio for the operation. If the noise to signal ratio is large, the error rate would be great in reading data from the disc. Consequently, the disc drive is not able to properly write and read data.
For thin film heads, wherein writing and reading are performed by the same transducer, fine head positioning can be accomplished by using a signal retrieved from the disc when information from the disc is read. The signal indicates when the transducer is positioned at its preferred location relative to the track, or "ideal track center"; which is typically the same for reading and writing.
Magnetoresistive heads (MR heads) are commonly referred to as dual element heads. An MR head has one transducer which is used to write data to the disc, and another transducer which is used to read data from the disc, i.e. a write transducer and a read transducer. MR heads have spatial separations between the read and write transducers. Additionally, read and write transducers on a single MR head can be spatially separated from one another to a greater or lesser degree than otherwise desired, based on manufacturing tolerances. Thus, when the MR head is finally positioned over a track during a write operation, that same position is not necessarily the ideal track center for the MR head during a read operation. In other words, the ideal center for operating the read transducer is not necessarily the same as the ideal center for operating the write transducer.
One method for providing head position information is to embed servo information in a sufficient number of sectors interleaved within the tracks such that servo information may be periodically sampled and held, and head position thereupon derived from the samples. To be effective, an embedded servo pattern should include information identifying the track as unique from its neighboring tracks, and the pattern should provide a centerline reference as well. The track identification number is useful during track seeking operations to indicate the radial position of the data head transducer relative to the disc surface, and the centerline reference is useful to center the data head transducer over the ideal track centerline during track following operations. Such servo information in the prior art has included a spatial quadrature relationship which may be used to indicate the direction of movement of the transducer relative to the tracks during seeking.
It is known that a data head transducer may function as a very accurate radial position measurement device relative to recorded patterns passing by the head transducer. If the head reads a prerecorded burst pattern, the amplitude of the recovered signal will be proportional to the degree of coincidence radially between the head transducer and the burst pattern. If the head is in alignment with the burst, a maximum amplitude is recovered. If only a fraction of the burst is encountered by the head, the amplitude of the recovered signal will be a fractional amount of full amplitude which is proportional to the radial displacement of the head. If the head misses the burst completely, then no burst amplitude is recovered.
With modern servo writing techniques, embedded sector servo patterns are typically written in multiple phase coherent passes of the data transducer head so as to record servo data field and centering burst patterns which are wider than the electrical width or head gap of the data head transducer. However, a head may be aligned completely with a servo burst, but incapable of resolving relative position within a dimension by which the radial width of the burst exceeds the head width. This dimension is in effect a servo dead zone. As the head moves throughout the dead zone, the amplitude of the signal recovered from the burst will remain substantially invariant. Thus, the servo loop experiences a dead band through this range.
The prior art has attempted to accommodate the dead band by providing two or four time-staggered, radially-offset adjacent bursts having burst edges of a pair of bursts aligned with the track centerline in each servo sector. The relative amplitudes of two selected bursts having opposite edges in alignment with the centerline of the track being followed are then compared to develop a centerline offset error signal. However, this prior approach has not provided accurate position information when the head is not in precise alignment between the two radially aligned edges of the time staggered bursts. This situation becomes important during track seeking operations and most particularly during a transitional operational phase between the track seeking mode and the track following mode, a phase known as track settling.
Moreover, an MR head typically requires operation at two positions within any given track. As such, the ideal MR head position is typically not in precise alignment with the edges of the bursts. Consequently, MR heads typically operate without precise alignment information, which leads to off-track errors. Thus, there is a substantial need to provide precise alignment information to an MR head.
Another drawback of the prior art in both MR and thin film heads is that typically the amplitude variance of each burst due to offsets or other irregularities creates errors and non-linearities in the position signal. For example, errors and non-linearities impede the ability of the drive to resolve head position when moving between bursts because the position signal does not exhibit monotonicity, and can include errors in the position signal of approximately 15%. These errors and non-linearities are exacerbated as track densities continue to increase, which require more precise and accurate positioning of the transducers. Thus, there is substantial need to overcome errors and non-linearities in the positioning of the data head in order to facilitate the increased track densities.